1. Field of the Invention
This invention relates generally to the production of hydrocarbons from subsurface formations and to in-hole seismic data acquisition to map advancing fluid fronts and the depletion of hydrocarbons within a hydrocarbon producing formation around a single well bore or outside a well bore on the surface of the earth or on the ocean bottom. The invention relates specifically to an autonomous carrier providing a movable obstruction, the carrier providing a seismic source or a discontinuity or obstruction to convert tube waves to body waves, the carrier being movable inside and outside the well bore and on the earth""s surface or on the ocean bottom.
2. Description of the Related Art
In order to relatively precisely map advancing fluid fronts within a field or around a single well bore requires the use of deep reading measurements at spatial resolutions of less than five (5) meters but with the spatial extents of several hundred meters, depending upon the reservoir location, size and the number of wells in the field. Conventional three dimensional (xe2x80x9c3Dxe2x80x9d) seismic acquisition and repeated 3D seismic acquisitions (also referred to as the 4D seismic acquisition) and seismic data acquisition techniques known as vertical seismic profiling (xe2x80x9cVSPxe2x80x9d), 3D VSP and Reverse VSP or Reverse 3D VSP are often utilized to model the reservoirs and/or to determine the advancing fluid fronts in the producing formations. The conventional 3D and 4D surface seismic acquisitions are performed by deploying detectors at or near the earth""s surface and the survey area is usually substantially large. The conventional 3D and 4D surveys provide data with limited spatial resolution and no near real-time ability to utilize results because of the lengthy time span required to acquire and process the data, which can take several months. The subsurface VSP and 3D VSP also suffer from long data processing cycles and have limited spatial extent. Water breakthrough can occur rapidly, especially after a new horizontal well is drilled. Reservoir engineers can take timely action if timely fluid front information is available.
Another related problem is the expense of acquiring repeat 3D seismic data over a relatively small geographical area, such as between 10-20 Km2. The current seismic surveying vessels using surface towed cables are designed to acquire vast volumes of data over a large region. Ocean bottom cable surveys, wherein seismic sensor or detector cables are deployed on the sea bottom, provide an alternative surveying method but are more expensive than the towed streamer cable acquisition methods.
The term xe2x80x9csignaturexe2x80x9d as used herein, means the variations in amplitude, frequency and phase of an acoustic waveform (for example, a Ricker wavelet) expressed in the time domain as displayed on a time scale recording. As used herein the term xe2x80x9ccodaxe2x80x9d means the acoustic body wave seismic energy imparted to the adjacent earth formation at a particular location. The coda associated with a particular seismic energy source point or minor well bore obstruction in this invention will be the seismic signature for that seismic energy source point. The term xe2x80x9cminor borehole obstructionxe2x80x9d, xe2x80x9cobstructionxe2x80x9d, xe2x80x9cborehole discontinuityxe2x80x9d or xe2x80x9cdiscontinuityxe2x80x9d means an irregularity of any shape or character in the borehole such that tube wave energy transiting the wellbore will impart some energy to the irregularity in the borehole and thus radiate body wave energy into the surrounding earth formation while also transmitting and reflecting some the tube wave energy as well.
The term xe2x80x9cimpulse responsexe2x80x9d means the response of the instrumentation (seismic sensors and signal processing equipment) to a spike-like Dirac function or impulse. The signal energy of an acoustic wave field received by seismic sensors depends upon the texture of the rock layers through which the wave field propagated, from which it was reflected or with which it is otherwise associated, whether along vertical or along lateral trajectories. The term xe2x80x9ctexturexe2x80x9d includes petrophysical parameters such as rock type, composition, porosity, permeability, density, fluid content, fluid type and inter-granular cementation by way of example but not by way of limitation.
From the above considerations, it is reasonable to expect that time-lapse seismic monitoring, that is, the act of monitoring the time-varying signature of seismic data associated with a mineral deposit such as an oil field over a long period of time, would allow monitoring the depletion of the fluid or mineral content, or the mapping of time-varying attributes such as the advance of a thermal front in a steam-flooding operation.
Successful time-lapse monitoring requires that differences among the processed data sets must be attributable to physical changes in the petrophysical characteristics of the deposit. This criterion is severe because changes in the data-acquisition equipment and changes in the processing algorithms, inevitable over many years may introduce differences among the separate, individual data sets from surveys that are due to instrumentation, not the result of dynamic reservoir changes.
In particular, using conventional surface exploration techniques, long-term environmental changes in field conditions such as weather and culture may affect the outcome. If time-lapse tomography or seismic monitoring is to be useful for quantitative oil-field reservoir monitoring, instrumentation and environmental influences that are not due to changes in reservoir characteristics must be transparent to the before and after seismic data sets. Successful time-lapse tomography requires careful preliminary planning.
One way to avoid many time-dependent environmental changes and updated state-of-the-art instrumental changes is to permanently install seismic sources and seismic detectors in one or more boreholes in and around the area of economic interest. Identical processing methods are applied to the data throughout the monitoring period using cross-well (cross-borehole) tomography rather than conventional surface type operations. One such method is disclosed in patent application Ser. No. 08/949,748, filed Oct. 14, 1997 and assigned to the assignee of this invention and which is incorporated herein by reference as a teaching of cross-well tomography.
U.S. Pat. No. 5,406,530, issued Apr. 11, 1995 to Tokuo Yamamoto, teaches a nondestructive method of measuring physical characteristics of sediments to obtain a cross sectional distribution of porosity and permeability values and variations and of shear modulus and shear strength. A pair of bore holes has bore hole entries spaced apart from each other at a predetermined distance and a plurality of hydrophones are spaced at predetermined known locations. A pseudo random binary sequence code generator as a source of seismic energy is placed in another bore hole and activated to transmit pseudo random wave energy from the source to the hydrophones. Seismic wave characteristics are measured in a multiplicity of paths emanating from the source to the hydrophones using cross-well tomography.
The Yamamoto teaching is primarily directed to use in shallow bore holes for engineering studies. Such holes are less than 100 meters deep, as opposed to oil-field bore holes, which may be two to five kilometers deep. The requirement for an active source to be placed at various levels in the bore hole is problematic because the source can damage the hole and interfere with production. Since the seismic equipment must be moved up and down the bore hole, it is difficult, using conventional methods, to precisely locate and/or position seismic equipment to maintain identical recording conditions over an extended time period.
G. W. Winbow in U.S. Pat. No. 4,993,001 issued Feb. 12, 1991, describes a method and apparatus for converting tube waves into down hole body waves for seismic exploration. The equipment comprises a rotary-valve tube wave source for producing swept-frequency tube waves that are injected into tubing or well bore fluid. The tube waves are converted into body waves by an elongate tube wave converter located at a selected position down hole. The tube wave converter comprises an elongate body that preferably substantially fills the well bore or tubing and has a preferred shape in order to efficiently convert the tube waves to body waves at the selected position downhole. This patent is directed primarily to reverse VSP. Winbow acknowledges that it is well known in the art that xe2x80x9cnonuniformities in the boreholexe2x80x9d cause seismic-wave mode conversions that cause secondary acoustic radiation and associated multiples.
Winbow employs a single tube-wave converter to serve as a single source of direct and reflected seismic waves but Winbow must repeatedly reposition the device at spaced-apart intervals down the length of the bore hole to obtain extended vertical coverage as in cross-well tomography. The Winbow system, thus is difficult to implement for the fixed permanent instrumental installation required for 4-D seismic monitoring operation.
There is a need for a passive system of benign seismic sources that can be easily and precisely moved around inside and outside of the bore hole on the earth""s surface or the ocean bottom and designed for monitoring time-varying reservoir attributes such as the distribution of the contents of a mineral deposit. There is also a need for an autonomous carrier for operation inside and outside of the borehole.
The present invention provides systems and methods for acquiring seismic data by deploying movable clusters of seismic obstructions and detectors in wells to generate acoustic signals and acquire data as needed. Such a system provides seismic data with relatively high spatial resolution and with small spatial extent. Because of the relatively small number of detectors and geometry of deployment, the data can be processed substantially in real-time and utilized to provide 4D maps of the advancing fluid fronts. Use of such systems in multiple wells in a common field can provide maps of the advancing fluid fronts and hydrocarbons within that field.
In one aspect, this invention provides a near real-time system and method for acquiring seismic data in reservoirs at very high spatial resolution such that advancing fluid fronts and hydrocarbons can be mapped substantially in real time. The system enables large spatial extents to be investigated at arbitrarily fine spatial intervals or resolution. In one system, one or more autonomous devices are deployed in the well to deploy movable obstructions, spaced to convert unique acoustic tube waves to body waves that radiate into a formation adjacent the well bore. Autonomous carriers deploy seismic receivers and may also deploy an acoustic energy source. The device may include multiple spaced-apart receivers. An acoustic source or naturally occuring fluctuations in production flow transmit energy into the well bore or production tubing to supply tube waves for conversion to body waves by the movable obstructions.
The autonomous devices move in the well and provide discontinuities that convert tube waves into a uniquie series of body waves that radiate into the formation. Seismic sensors or receivers detect seismic waves traveling to the receivers at known discrete locations in the well. The receiving devices store the seismic data in on-board memory. After the data acquisition, the devices dock at receiver stations in the well. The receiver stations provide power to the devices and download the stored data from the memory. A two-way data link between a surface control unit, such as a computer system, and the down hole receiver is used to transmit data from the receiver to the surface computer. The surface computer system also sends command signals to the down hole carriers to control the location and operation of the individual devices deployed on the carriers. The receiver stations are programmed to control the operation of the devices deployed on the carriers, which may include resident programs to perform the survey operations at specified intervals.
The data gathered by the devices is used to update existing seismic maps in determining the boundary conditions of the fluid fronts and the depletion of hydrocarbons. The amount of seismic data is relatively small compared to conventional seismic methods, such as surface seismic methods using land cables or streamer cables. Thus, the data can be processed to update the prior 3D data to locate fluid fronts and hydrocarbons, substantially in real time. The data collection spacing defines the spatial resolution, which is selected by the operator based upon the needed resolution.
In an alternative method, autonomous carriers are deployed in the well bore and at or near the sea bottom. The devices travel along predefined paths at the sea bottom and inside and outside of the wells carrying seismic sources, providing discontinuities inside the well bore, to convert tube waves into body waves, providing seismic sources or providing receivers for collection of seismic data. Tracks are used to guide the autonomous devices in the wells and along the ocean bottom. Coiled tubing laid at the ocean bottom may be used as tracks. A subsea control station or receiver station provides power and data transmission function for the subsea devices. A source on a vessel may be used to induce acoustic tube wave energy into the subsurface formations. The data from both the wells and the sea bottom is then used to update the 3D maps to obtain 4D maps and to model the reservoirs.
The present invention provides an autonomous carrier acoustic system and method for providing an acoustic source or a moveable obstruction for radiating a wave field into an earth formation surrounding at least a first source bore hole containing a column of well bore fluid. A movable acoustic driver is provided for exciting a reverberating tube wave in the column of well bore fluid. In the alternative the naturally occurring production flow fluctuations provide a tube wave. A plurality of movable autonomous carriers are arbitrarily spaced in the bore hole to provide discontinuities or minor obstructions or irregularities in well bore fluid column. The present invention can be utilized to deploy a plurality of moveable obstructions can be deployed simultaneously in a plurality of wells in an oil field. See, e.g., U.S. Pat. No. 5,886,255. The autonomous carriers also carry seismic sources or other devices inside or outside of the borehole. Each autonomous carrier inside the borehole or production tubing provides a discontinuity or minor bore hole obstruction that forms a point source for diffractively radiating an acoustic wavefield into the surrounding formation. The discontinuities or minor obstructions intercept the tube wave as the tube wave transits the column of well bore fluid or the fluid inside of a production tubing. The resulting plurality of acoustic wave fields, each exhibiting a unique waveform coda, is thereby radiated into the formation. An acoustic sensor in association with one or more said discontinuities or minor bore hole obstructions detects the unique waveform coda characteristic of each of the respective wave fields to provide a plurality of uniquely encoded pilot signals. In a preferred embodiment, a sensor is collated with a movable obstruction source to determine the coda associated the with the movable obstruction.
A plurality of sensors for detecting seismic energy separated from the source borehole by an intervening earth formation is provided. The plurality of seismic receivers, for receiving the plurality of acoustic wavefields radiated from the source borehole after the wavefields have propagated through the intervening earth formation, may be in another borehole or on the surface of the earth. The receivers are also carried on autonomous carriers. A cross correlator is provided for cross correlating the respective uniquely encoded pilot signals with a corresponding received acoustic wavefield to furnish a cross correlogram that is indicative of the current value of the preselected attribute. A time-lapse profile is created that is representative of the temporal changes in the petrophysical characteristics and mineral content or distribution of the intervening earth formation by repeating the process at subsequent time intervals.
Examples of the more important features of the invention thus have been summarized rather broadly in order that the following detailed description thereof may be better understood, and in order that the contributions to the art may be appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form the subject of the claims appended hereto.